In a vehicle, the front wheels which are usually main steered wheels are disposed steerably in a desired running direction of the vehicle, while the rear wheels which are subsidiary steered wheels are disposed parallel to the longitudinal direction of the vehicle. When the front wheels of such vehicle are steered and turned, the front wheels and rear wheels do not match in the turning circle, and the rear wheels get inside of the turning circle due to inner wheel difference when the vehicle turns at low speed, while the front wheels get inside of the turning circle due to centrifugal force when the vehicle turns at high speed. Accordingly, when the front wheels are steered in the steering direction which is the running direction of the vehicle, the position of the vehicle cannot be matched with the turning direction.
A vehicle was hence proposed to have a four-wheel steering device for enhancing the traveling performance by steering not only the front wheels but also the rear wheels. That is, in this four-wheel steering device, a main turning means for steering front wheels and a subsidiary turning means for steering the rear wheels are provided in order to improve the recovery of the vehicle when parking in a garage or turning a narrow road at low speed, and enhance the steering stability when changing lanes at medium or high speed.
Such four-wheel steering device is disclosed, for example, in Japanese Laid-Open Patent Sho. 61-18569. What is disclosed in this publication is intended to prevent the steering angle of the rear wheels from being changed due to external force when the rear wheels are positioned near the neutral position, by suppressing the movement of the steering rod for the rear wheels by a friction mechanism when the rear wheels are positioned near the neutral position.
Various four-wheel steering devices have been devised which, for example, increase the degree of freedom for mounting by downsizing the subsidiary turning means for steering the rear wheels, or turn the rear wheels by using a motor as the driving source to realize an oil-free operation.
A four-wheel steering device using a motor is disclosed, for example, in Japanese Laid-Open Patent Sho. 61-202977 (and corresponding U.S. Pat. No. 4,645,025). What is disclosed in this publication is to define the shift of the steering rod for the rear wheels by a stopper mechanism or the like, while holding the rear wheels in the neutral position by a neutral position holding mechanism composed of a preset spring and others when external force is not applied on the rear wheels.
In the conventional four-wheel steering devices, however, when the vehicle is traveling at high speed and the rear wheels are rolling, if the power source to the motor is shut off due to trouble in the motor control system or the like, the rear wheels are suddenly returned to the vicinity of the neutral position by the external force and the action of a return spring, and there suddenly occurs a vehicle behavior change that the driver cannot cope with and which is very dangerous. In particular, while the rear wheels are steered in the same direction as the front wheels, and if the rear wheels are suddenly returned to the neutral position, the vehicle is suddenly oversteered, and a so-called tuck-in phenomenon occurs which can lead to spinning in a worst case.
It is hence a primary object of this invention to provide a four-wheel steering device for an automobile which results in enhanced safety by improving upon the above-discussed problems.
It is a further object of the invention to provide a four-wheel steering device which converges the steering angle of the subsidiary steered wheels gradually to zero in the event of abnormality of the motor control system to return the subsidiary steered wheels to the neutral position without causing sudden behavior change of the vehicle, while preventing abnormal control of the subsidiary steered wheels due to trouble in the control system by steering the vehicle only by the two main steered wheels, and which transfers smoothly from four-wheel steering to two-wheel steering.